Apparatus for producing single crystals including halogen lamps aligned with the common major axes of a spheroidal reflector pair

ABSTRACT

For an apparatus for growing single crystals by the floating zone method, a pair of congruent prolate spheroidal reflectors for radiant energy are disposed outwardly of each other with their major axes aligned and with the distance between their axially inwardly disposed foci being less than the diameter of a molten zone placed at the location of these foci. A pair of congruent halogen lamps for supplying the radiant energy are held inside the respective spheroidal surfaces with their logitudinal axes parallel to the major axes and with the center of each lamp placed, apart from the axially outwardly disposed focus, less than a quarter of the longitudinal dimension of the lamps and a half of the transverse dimension thereof in the directions parallel and perpendicular to the major axes, respectively.

United States Patent Mizutani et al.

[ APPARATUS FOR PRODUCING SINGLE CRYSTALS INCLUDING HALOGEN LAMPSALIGNED WITH THE COMMON MAJOR AXES OF A SPHEROIDAL REFLECTOR PAIR [75]Inventors: Takayuki Mizutani; Tsutomu Yamamoto, both of Tokyo, JapanNippon Electric Company, Limited, Tokyo, Japan Filed: Feb. 2, 1972 Appl.No.: 222,777

[73] Assignee:

[30] Foreign Application Priority Data [5 6] References Cited UNITEDSTATES PATENTS 5/1972 Morrone .1. ..2l9/349X 3,427,435 2/1969 Webb219/347 X [451 Sept. 25, 1973 FOREIGN PATENTS OR APPLICATIONS 498,5013/1930 Germany 219/347 OTHER PUBLICATIONS Fixture For Infrared Sealingof Metal to Glass, Hentz et al., Western Electric Technical Digest No.3, July, 1966, pages 15 and 16.

Primary Examiner-A. Bartis Att0meyNichol M. Sandoe et al.

[57] ABSTRACT For an apparatus for growing single crystals by thefloating zone method, a pair of congruent prolate spheroidal reflectorsfor radiant energy are disposed outwardly of each other with their majoraxes aligned and with the distance between their axially inwardlydisposed foci being less than the diameter of a molten zone placed atthe location of these foci. A pair of congruent halogen lamps forsupplying the radiant energy are held inside the respective spheroidalsurfaces with their logitudinal axes parallel to the major axes and withthe center of each lamp placed, apart from the axially outwardlydisposed focus, less than a quarter of the longitudinal dimension of thelamps and a half of the transverse dimension thereof in the directionsparallel and perpendicular to the major axes, respectively.

' i lil lllll vIOZ- lildl' APPARATUS FOR PRODUCING SINGLE CRYSTALSINCLUDING HALOGEN LAMPS ALIGNED WITH THE COMMON MAJOR AXES OF ASPHEROIDAL REFLECTOR PAIR BACKGROUND OF THE INVENTION This inventionrelates to an apparatus for growing single crystals by the floating zonemethod with radiant energy supplied from a pair of halogen lamps andconcentrated onto the sample floating zone by a pair of inwardlyreflecting, prolate spheroidal surface portions.

In Japanese Pat. application No. Syo 45-93,850 filed Oct. 23, i970, andU.S. Pat. application Ser. No. 173,458 filed Aug. 20, 1971, an apparatusfor heating a sample with concentrated radiant energy is described whichcomprises a pair of inwardly reflecting, substantially prolatespheroidal surface portions disposed outwardly of each other, saidspheroidal surface portions having aligned major axes and a commonfocus, a source of radiant energy placed at one of the two foci of saidspheroidal surface portions that are conjugate to said common focus, andmeans for holding a sample at the other of the two conjugate foci ofsaid spheroidal surface portions. The sample may be typically apolycrystalline rod that is subjected to the floating zone method to begrown into a single crystal. In Japanese Pat. application No. Syo45-95,37l filed Oct. 28, 1971, and the above-mentioned United Statespatent application, a similar apparatus is disclosed which furthercomprises a second, similar pair of prolate spheroidal surface portionsdisposed outwardly of each other and of the first-mentioned pair ofspheroidal surface portions, one of the two foci of said second pair ofspheroidal surface portions that are conjugate to the common focus ofthese spheroidal surface portions being placed at the above-mentionedother focus of the two conjugate foci of the first-mentioned pair ofspheroidal surface portions, and a second source of radiant energyplaced at the other of the two conjugate foci of said second pair ofspheroidal surface portions. As described in the latter twoapplications, each source of radiant energy may be a helically woundfilament of a halogen lamp disposed perpendicular to the aligned majoraxes; r

In Japanese Pat. application No. Syo 45-82,438 filed Sept. 18, l970,anda corresponding U.S. Pat. application Ser. No. 179,714, filed Sept. 13,1971, an apparatus for producing a single crystal by the floating zonetechnique is disclosed which includes a heating device comprising aprolate spheroidal reflector and a halogen lamp placed at one focus ofsaid reflector, the light emitted by said lamp being concentrated bysaid reflector at the other focus of said reflector and which ischaracterized in that said reflector has a ratio of the minor diameterto the major diameter in the range of 0.87 to 0.96 and that saidapparatus further comprises a tube of a refractory material translucentfor said light for accommodating the floating zone and its vicinity andfor substantially defining a chamber together with said reflector andmeans for ventilating said chamber at a rate of at least once every 2seconds.

The invention revealed in the first-mentioned three applications isdirected to an apparatus whereby the radiant energy is concentrated onan image that is in the best possible congruency with the source orsources of radiant energy. It is, however, desirable to simplify thedesign of the apparatus and to lengthen the lift of the halogen lamp orlamps employed as the radiation source means. Furthermore, a restrictedhigh temperature zone in the direction perpendicular to the alignedmajor axes should be provided in some applications, such as applicationto the floating zone technique. The primary object of the inventiondescribed in the lattermentioned applications is to lengthen the life ofthe halogen lamp and to attain as uniform an azimuthal temperaturedistribution as possible around the crystal being grown. It is, however,desirable to further improve the azimuthal temperature distribution. Inaddition, the amount of the radiant energy concentrated at the samplemust be increased in order to obtain a large single crystal and/or asingle crystal of a higher melting point substance.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide an apparatus for growing single crystals by thefloating zone method with concentrated radiant energy supplied fromhalogen lamps that is simple in design.

It is another object of this invention to provide an apparatus of thetype described that enables a large amount of the radiant energy to beconcentrated at the molten zone.

It is still another object of this invention to provide an apparatus ofthe type described that enables the radiant energy to be concentrated atthe molten zone with restricted longitudinal distribution and withsubstantially uniform azimuthal distribution.

It is yet another object of this invention to provide an apparatus ofthe type described wherein the life of each radiant energy source meansis an elongated halogen lamp.

According to the instant invention there is provided an apparatus forheating a molten zone with concentrated radiant energy including a pairof inwardly reflecting, substantially prolate spheroidal surfaceportions disposed outwardly of each other having their major axessubstantially aligned with each other, and a pair of elongated sourcesof radiant energy wherein the improvement comprises:

means for holding said spheroidal surface portions in such a manner thatthe distance between the axially inwardly disposed foci of saidspheroidal surfaces is less than the dimension of said sample in thedirection of said aligned major axes;

means for holding said elongated sources of radiant energy (halogenlamps) inside said spheroidal surfaces, respectively, in such a mannerthat the longitudinal axes of said sources are substantially parallel tosaid aligned major axes, and that the center of each said source isspaced from the axially outwardly disposed focus of the associatedspheroidal surfaces, by less than one quarter of the longitudinaldimension of the source when measured in a direction parallel to thealigned major axes-and less than one half of the transverse dimension ofthe source when measured in the direction perpendicular to said alignedmajor axes and means for holding said molten zone in a position suchthat said radiant energy converging toward at least one of said axiallyinwardly disposed foci is incident on the surface of said sample.

According to a preferred aspect of this invention, the apparatus furthercomprises a tube of a refractory material translucent for said radiantenergy for accommodating said molten zone and for substantially defininga chamber together with said spheroidal surface portions, said tubebeing disposed perpendicular to said aligned major axes, and stillfurther comprises means for forcibly ventilating said chamber.

BRIEF DESCRIPTION OF THE DRAWINGS For a more detailed understanding ofthe invention, reference may be had to the description below taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic side view of an embodiment of the presentinvention, with the heating means shown in axial vertical section;

FIG. 2 shows the radiant energy distribution around a halogen lamp;

FIG. 3 shows the azimuthal energy distribution around the molten zone;and

FIGS. 4 and 5 are fragmentary axial vertical sectional views of modifiedholding means for the halogen lamps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, anapparatus according to the present invention for growing a singlecrystal by the floating zone method comprises a pair of inwardlyreflecting, substantially prolate spheroidal surface portions 1 and 1'disposed outwardly of each other and having mutually mating circularends (not shown). The prolate spheroidal surfaces are preferablycongruent and have substantially aligned major axes and a substantiallycommon inwardly disposed focus F or F The inwardly reflecting surfacesare preferably covered with gold either by'plating or by evaporationwith a view to raising the stability of the surfaces. Two elongatedsources of radiant energy 2 and 2 are placed at the outwardly disposedfoci F and F, of the spheroidal surfaces which are conjugate to thesubstantially common focus F or F Each of the sources 2 and 2' ispreferably a quartz halogen lamp having a helically wound tungstenfilament 3 or 3 and an accompanying power supply and holding means 4 or4' and 5 or 5'. The filements 3 and 3' are the actual sources of the ra'diant energy. As will be described in detail below, it has nowunexpectedly been found that an appreciable increase in the heatingcapability and the desired distribution of the radiant energy around thesubstantially common focus F, or F, are achieved when the axes of thefilaments 3 and 3' are held substantially parallel with the alignedmajor axes with the centers of the filaments 3 and 3 placedsubstantially at the respective foci F and F A seed crystal 11 is heldby a lower chuck 12 attached to a vertical lower shaft 13. A rod of rawmaterial or a polycrystalline rod 14 is similarly held by an upper chuck15 attached to a substantially aligned vertical upper shaft 16. With thebottom surface of the polycrystalline rod 14 brought into contact withthe top surface of the seed l1 and with the sources 2 and 2 excited, theabutting surface portions of the seed 11 and the rod 14 are heated bythe radiant energy concentrated at the substantially common focus F, orF, to form a molten zone 17.

In the manner of the known floating zone technique and principles, thelower shaft 13 is rotatably and vertically slidably supported by a lowerbearing means 21 and driven by a lower motor 22 through a pulley 23, abelt 24, and another pulley 25 as indicated by an arrow 100. Likewise,the upper shaft 16 is rotatably and vertically slidably supported by anupper bearing means 26 and driven by an upper motor 27 through a pulley28, a belt 29, and another pulley 30 in the direction indicated by anarrow 101. Furthermore, the driving means for the lower and the uppershafts l3 and 16 are carried by a vertically movable support 31 having afemale screw 32 engaging a male screw 33 that is driven through areduction gear 34 by a feed motor 35 to feed the support 31 very slowlyalong guide rails 36 and 36', with the sliders 37, 37', 37", and 37 ofthe support 31 sliding therealong. The rotation given to the molten zone17 not only improves the azimuthal uniformity of heating but alsoprovides the stirring action which in turn improves the homogeneity ofthe composition within the molten zone 17. Except during the initialmounting of the seed 11 and the polycrystalline rod 14, the lower andthe upper shafts l3 and 16 are carried by the support 31 with therelative position unchanged and are fed downwardly relative to theheating means so that the molten zone 17 may travel upwardly relative tothe polycristallyne rod 14 leaving the growing single crystal on theseed 11.

The apparatus further comprises a quartz tube 40 passing through thecongruent spheroidal surface portions 1 and 1' adjacent to the matingends. The tube 40 accommadates the molten zone 17 and its adjacent areaand surrounds the adjacent end portions of the lower and the uppershafts 13 and 16. It is provided at the lower and the upper ends with agas inlet 41 and a gas outlet 42, respectively. Sufficient clearancesfor the thermal expansion should be left between the tube 40 and theprolate spheroidal surface portions 1 and l'. The tube 40 and theprolate spheroidal surface portions 1 and 1 thus define a substantiallyannular chamber. The apparatus further comprises pipes 51, 52, 53, and54 for ventilating the chamber and ducts 61 and 61' for cooling theprolate spheroidal reflectors 1 and 1'.

Referring to FIG. 2 wherein the abscissa represents the angle 0schematically shown therein for a halogen lamp and the ordinaterepresents the relative radiation energy in percentage, it is understoodthat most of the radiant energy is directed within a certain solid angleformed on both sides of a plane bisecting the length of the filament. Itfollows therefore that in an apparatus of the kind wherein the elongateradiant energy sources are disposed perpendicular to the aligned majoraxes, an appreciable amount of the radiant energy is directed within asolid angle subtended by the mating ends of the prolate spheroidalsurface portions 1 and 1' without being concentrated at thesubstantially common focus F or F,. In accordance with the instantinvention, the loss of radiant energy is reduced to a minimum tostrengthen the heating capability.

With two 2-kW quartz halogen lamps disposed parallel to the alignedmajor axes, the radiant energy concentrated at a cylindrical sample of10 mm in diameter was 1,950 W (49 percent). With two similar lampsdisposed perpendicular to the aligned major axes and to the sample, theabove-mentioned radiant energy was 1,700 W (43 percent). With onesimilar lamp used in the apparatus described in the aforecited latterapplications, the above-mentioned radiant energy was 1,420 W (71percent). In this connection, it should be pointed out that the quartztube 40 absorbs about 10 percent of the radiant energy incident thereon.At any rate, it is possible with the instant invention to increase theamount of the radiant energy concentrated at the sample although thehighest efficiency is attainable with theone-prolate-spheroidal-reflector apparatus due to the fact that almostall radiant energy emitted by the radiant energy source placed at one ofthe foci is concentrated at the other focus. It is thus possible with anapparatus according to this invention to grow a single crystal as largeas l5 mm in diameter of a manganese-zinc ferrite whose melting point isl,650 C while the maximum diameter of the crystal grown by theone-prolate-spheroidal-reflector apparatus is 12 mm. Furthermore, it ispossible with an apparatus according to this invention to melt a rod ofmm in diameter of calcium titanate having the melting point of l,9l5 C.

Referring to FIG. 3 wherein the abscissa represents the azimuth aroundthe cylindrical molten zone 17, a solid-line curve 71 shows the relativeradiant energy incident on the peripheral surface of the molten zone 17as simulated by an electronic computer with respect to an apparatus inaccordance with the present invention. A dashed-line curve 72illustrates the similar energy as likewise simulated in respect of aone-prolatespheroidal-reflector apparatus. The latter-mentionedapparatus gives rise to an azimuthally periodec temperature variation ofas much as i0.6 C in the molten zone 17 even though the zone 17 isrotated at a rate of 30 RPM. With an apparatus according to thisinvention, it is possible to reduce the temperature variation to i0.l Cat most with the molten zone 17 rotated at 30 RPM.

Simulation by means of an electronic computer has further revealed thatthe total amount of the radiant energy incident on the molten zone andthe aximuthal distribution of such energy is nevertheless acceptableeven if the distance between the closely spaced foci F, and F, is lessthan the dimension of the molten zone in the direction of the alignedmajor axes and if the center of each elongated halogen lamp is spacedfrom the focus F or F by an amount less than a quarter of thelongitudinal dimension of the lamp, or more exactly, the length offilament coil in the direction parallel with the aligned major axes andlessthan a half of the diameter of the source in the directionperpendicular to the aligned major axes. The center of the halogen lampmay be displaced either inwardly or outwardly relative to the focus F orF in the direction parallel to the aligned major axes. Similarly, thecenter of the halogen lamp may be displaced relative to the focus F or Falong a plane perpendicular to the aligned major axes.

The ventilating means serves to keep the quartz tubes of the halogenlamps 2 and 2' within the desired temperature range of 200 C throughl,0O0 C. Otherwise, the filaments 3 and 3 will snap within severalminutes after being switched on due to the disturbance caused to thehalogen cycle because the quartz tubes of the lamps 2 and 2' are heatedabove l,300 C on account of the additional heating resulting from theradiant energy concentrated back from the molten zone 17 at the halogenlamps 2 and 2'. It is thus possible to keep the quartz tubes of thelamps 2 and 2' at about 300 C and to lengthen the life to about 100hours by pumping the air at l X 10 litres an hour into or out of theabovementioned chamber of the volume of about six liters through thepipes 51, 52, 53, and 54. Substitution of an inactive gas, such asnitrogen, for the air attains an additional advantage of preventingoxidation of the refleeting surfaces. The gas may be pumped into and outof the chamber through some of the pipes 51, 52, 51, and 54 and theremaining pipes, respectively. In this connection, it is to be notedthat localized cooling of the quartz tubes of the lamps 2 and 2', whichmay be effected by blowing the gas directly against the halogen lamps 2and 2, is not desirable because of the adverse effect caused thereby tothe halogen cycle.

Referring finally to FIGS. 4 and 5, the power supply and holding means 4or 4 which extends near to the tube 40 may be formed into the shapesdepicted therein. In any event, these supply and holding means aresubjected to the heat from the halogen lamp and the molten zone 17 andshould consequently be made of a refractory material. With thearrangement shown in FIG. 1, it is possible to effectively cool thepower supply and holding means 4 and 4 by circulating cooling watertherethrough as symbolically depicted'by an arrow 102.

With the apparatus illustrated with reference to FIG. 1, about a half ofthe energy emitted by the halogen lamps 2 and 2' is absorbed by themolten zone 17, the seed Ill, and the polycrystalline rod 14. Anappreciable amount of the absorbed energy is reradiated from thevicinity of the molten zone 17. Most of the reradiated energy and theother half of the energy emitted by the halogen lamps 2 and 2 isabsorbed by the prolate spheroidal surface portions 1 and 1' while it isrepeatedly reflected, thus raising the temperature thereof. The ducts 61are provided and 61 for a coolant, such as water to, prevent the prolatespheroidal surface portions 1 and 1' from being oxidized by theotherwise excessively raised temperature this would cause a reduction inreflecting power.

Incidentally, it is possible to leave a gap between the adjacent ends ofthe prolate spheroidal surface portions 1 and l. The tube 40 foraccommodating the molten zone may be made of any refractory materialthat is translucent to the radiant energy. It will be understood bythose skilled in the art that numerous variations and modifications ofthe embodiments disclosed above are possible. However, the scope of theinvention is defined by the following claims only and not by thedescription above.

We claim:

1. An apparatus for growing single crystals with radiant energy suppliedfrom a pair of elongated halogen lamps and concentrated by cooperationof a pair of inwardly reflecting, substantially prolate spheroidalsurface portions disposed outwardly of each other having their majoraxes substantially aligned with each other, wherein the improvementcomprises:

means for holding said spheroidal surface portions in such a manner thatthe distance between the axially inwardly disposed foci of saidspheroidal surfaces is less than the dimension of said molten zone inthe direction of said aligned major axes;

means for holding said lamps inside said spheroidal surfaces,respectively, in such a manner that the longitudinal axes of said lampsare substantially parallel to said aligned major axes, and that thecenter of each said lamp is spaced from the axially outwardly disposedfocus of the associated surfaces by less than one quarter of thelongitudinal dimension of the lamp when measured in an inward or outwarddirection parallel to the aligned major axes and less than one-half ofthe transverse dimension of the lamp when measured along a planeperpendicular to said aligned major axes from said aligned major axes;and

means for holding said molten zone in a position such that said radiantenergy converging toward at least one of said axially inwardly disposedfoci is incident on the surface of said molten zone.

2. The apparatus as claimed in claim 1, further comprising:

a tube of a refractory material that is translucent with respect to saidradiant energy for accommodating said molten zone and for substantiallydefining a chamber together with said spheroidal surface portions, saidtube being disposed perpendicular to spheroidal surface portions havecommon mating ends.

1. An apparatus for growing single crystals with radiant energy suppliedfrom a pair of elongated halogen lamps and concentrated by cooperationof a pair of inwardly reflecting, substantially prolate spheroidalsurface portions disposed outwardly of each other having their majoraxes substantially aligned with each other, wherein the improvementcomprises: means for holding said spheroidal surface portions in such amanner that the distance between the axially inwardly disposed foci ofsaid spheroidal surfaces is less than the dimension of said molten zonein the direction of said aligned major axes; means for holding saidlamps inside said spheroidal surfaces, respectively, in such a mannerthat the longitudinal axes of said lamps are substantially parallel tosaid aligned major axes, and that the center of each said lamp is spacedfrom the axially outwardly disposed focus of the associated surfaces byless than one quarter of the longitudinal dimension of the lamp whenmeasured in an inward or outward direction parallel to the aligned majoraxes and less than one-half of the transverse dimension of the lamp whenmeasured along a plane perpendicular to said aligned major axes fromsaid aligned major axes; and means for holding said molten zone in aposition such that said radiant energy converging toward at least one ofsaid axially inwardly disposed foci is incident on the surface of saidmolten zone.
 2. The apparatus as claimed in claim 1, further comprising:a tube of a refractory material that is translucent with respect to saidradiant energy for accommodating said molten zone and for substantiallydefining a chamber together with said spheroidal surface portions, saidtube being disposed perpendicular to said aligned major axes; and meansfor forcibly ventilating said chamber.
 3. The apparatus as claimed inclaim 2, wherein said ventilating means comprises means for introducinggas into said chamber.
 4. The apparatus as claimed in claim 2, whereinsaid ventilating means comprises means for sucking gas out of saidchamber.
 5. The apparatus as claimed in claim 1, wherein said spheroidalsurface portions are substantially congruent and said lamps aresubstantially congruent.
 6. The apparatus as claimed in claim 5, whereinsaid spheroidal surface portions have coMmon mating ends.